1. Field of the Invention
This invention relates to a passive seat belt system mounted in a vehicle such as an automotive vehicle, and more particularly to the passive seat belt system arranged to prevent a seat belt from being driven forward to reach its passenger releasing position during vehicle cruising even under malfunction of a control circuit due to various noise from the outside of the vehicle, thereby improving safety of a vehicle passenger.
2. Description of the Prior Art
A variety of passive seat belt systems have been proposed and put into practical use in the field of automotive vehicles. One of them is disclosed in Japanese Patent Provisional Publication No. 56-71643 in which a seat belt is provided to restrain the breast section of a vehicle passenger in the vehicle. The seat belt is engaged at its lower end section with a retractor disposed on the floor of the vehicle. The upper end section of the seat belt is movably engaged with and movable selectively forward and rearward along a roof rail extending in the fore-and-aft direction of the vehicle, in which the upper end section is moved forward when the door is opened thereby to facilitate seating action of the passenger while moved rearward when the door is closed thereby to restrain the passenger.
However, in such a conventional seat belt system, a control section for controlling operation of the seat belt usually employs a microcomputer and carries out complicated control at high speed, and therefore there is a possibility of making runaway of control program when strong noise is applied to the control section from the outside of the vehicle during vehicle cruising. This may drive the upper end section of the seat belt forward although the vehicle door is closed thereby unavoidably releasing the passenger from restraint by the seat belt, thus degrading safety for the vehicle passenger.